1. Field of the Invention
This invention relates to an apparatus for sorting and feeding bead supporters for an electron gun of a cathode-ray tube. More particularly, the invention is an apparatus for sorting and feeding bead supporters to an assembly line in a properly oriented posture suitable for assembly. The bead supporters support a plurality of cathode assemblies for an electron gun and the assemblies to opposing bead glasses.
2. Description of the Prior Art
Generally, an electron gun for a cathode-ray tube is of the type shown in FIG. 1 of the accompanying drawings. The electron gun comprises three equally spaced coplaner cathode assemblies 10 each including a cathode 4, a heater 5 and an eyelet 6. Each cathode assembly is supported by a bead supporter 7. First, second and third grids, 1, 2 and 3, are arranged in order ahead of each cathode assembly and are fixed to opposing bead glasses 14 at predetermined intervals.
Since cut-off voltages in the cathode assemblies are not always equal to one another, bias voltages which differ from one another must be applied to the respective cathode assemblies in order to obtain a constant electric current. Therefore, to obtain a good quality picture image on the screen of the cathode ray tube, it is desirable to make cathode currents of the respective cathode assemblies 10 equal to one another within a short period of time-after commencing operation.
It is desirable that the coplaner cathode assemblies 10 be kept equally spaced from the first grid 1 so as to obtain a good quality picture image. During operation of the electron gun, however, the distance between the first grid 1 and each cathode assembly 10 may vary as the cathode assembly and adjacent structures are heated by the heater 5, thereby adversely affecting the picture image on the screen. More specifically, during operation of the heater 5 of the electron gun, the cathode assembly 10 is heated to a temperature of about 800.degree. C. and the eyelet 6 is heated to a temperature of about 400.degree. C. As a result, the length of the cathode assembly extends from its initial set value by a value of EQU .DELTA.l(.DELTA.l=.alpha..multidot.l.multidot..DELTA.T)
where,
.alpha.: thermal expansion coefficient of the cathode assembly; PA1 l: length of the cathode assembly; PA1 .DELTA.T: difference between the normal temperature of the cathode assembly and the temperature of the assembly when heated by the heater.
The centrally located cathode assembly 10 is heated about 20.degree. C. higher than two outer cathode assemblies (not shown), however, due to radiant heat. Therefore, as shown in FIG. 2, the length A of the central cathode assembly 10 becomes longer than the corresponding length of the outer cathode assemblies, resulting in changes in distances between the respective cathode assemblies and the first grid.
In order to compensate for the changes in distances, for example, the head supporter 7 is either made of a material having a thermal expansion coefficient greater than the thermal expansion coefficient of the eyelet 6, or is formed to have an expanded length greater than the expanded length B of the eyelet. In this way variations in the expanded lengths of the central and outer cathode assemblies can be compensated, thereby maintaining equal distances between the first grid and the respective cathode assemblies.
In order to adjust the expanded lengths of the bead supporters 7 and the eyelets 6, the bead supporters are each formed with a plurality of projections 7a to allow the assembling positions of the supporters to the eyelets to be determined. More specifically, to compensate for the expanded lengths of the bead supporters 7, the bead supporters for supporting the outer cathode assemblies have the projections provided along their longitudinal central lines. In contrast, bead supporters for supporting the central cathode assembly have projections formed on a lower portion below a longitudinal central line.
During the assembly process of the electron gun, the bead supporters are transported by an assembly line. The bead supporters for the outer cathode assemblies can be transported without being sorted because each supporter, which has projections arranged along its longitudinal central line, is of a symmetrical configuration. This means that the projections of bead supporters for the outer cathode assemblies will be in the same position regardless of whether the bead supporters are transported in a normal or reverse position. In contrast, the bead supporter 7 for the central cathode assembly 10, which has the projections 7a formed on the lower portion below its longitudinal central line, is of an asymmetrical configuration. This bead supporter must, therefore, be sorted and transported by an apparatus which detects normal and reverse positions, and then transports only a normally positioned supporter to the assembly line.
According to the prior art, when the bead supporters 7 each having the projections 7a formed on the lower portion below its longitudinal central line, as shown in FIG. 5, are transported to the assembly line, they are first supplied from a parts feeder (not shown) to a bead supporter sorting and feeding apparatus. The apparatus comprises a parts feeding pin 8, a position detecting pin 9 and an optical sensor (not shown) positioned at a feed passage for the feed supporters 7, and grippers 13 for temporarily stopping the bead supporter, as shown in FIG. 3.
When the optical sensor detects that the bead supporter 7 has reached a given location for normal position detection, the grippers 13, which are freely movable, are moved to a forward position to temporarily stop the bead supporter. In this state, as shown in FIG. 4, the position detecting pin 9 is extended upwardly from its retracted position below the stopped supporter 7 through a central passage 15 of the supporter to detect the position of the projections 7a on the inner wall of the central passage 15. Through contact with the projections, the bead supporter is recognized as being either in a normal position or a reverse position. When the supporter is in the normal position, it is transferred to the next process for joining with the eyelet. If the bead supporter is in the reverse position, however, it is turned in the reverse direction by a shutter 16, as shown in FIG. 3, and positioned in the normal direction. It is then transferred to the next process.
The prior art apparatus which sorts the bead supporters by detecting the positions of the projections of each supporter has drawbacks, however, in that is complex in construction and results in higher manufacturing costs. Since the projections 7a of the bead supporter are very small, when foreign matter sticks to the projections, or the bead supporter is gripped in a tilted condition, or the extended position detecting pin is tilted even slightly, the position of the supporter may be erroneously detected.